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Facts about Fog for Kids

Fog is a collection of liquid water droplets or ice crystals suspended in the air at or near the Earth’s surface. The term “fog” is typically distinguished from the more generic term “cloud” in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or the ocean, or from nearby moist ground or marshes).

  • Fog is distinguished from mist only by its visibility, as expressed in the resulting decrease in visibility: Fog reduces visibility to less than 1 km (5/8 statute mile), whereas mist reduces visibility to no less than 1 km.
  • Even in generally warmer southern Europe, thick fog and localized fog is often found in lowlands and valleys, such as the lower part of the Po Valley and the Arno and Tiber valleys in Italy or Ebro Valley in northeastern Spain, as well as on the Swiss plateau, especially in the Seeland area, in late autumn and winter.
  • Fog begins to form when water vapor condenses into tiny liquid water droplets in the air.
  • Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds.
  • This can be achieved by either adding moisture to the air or dropping the ambient air temperature.
  • Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast.
  • Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon.
  • Fog commonly produces precipitation in the form of drizzle or very light snow.
  • Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.
  • The thickness of fog is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layer, above which the airmass is warmer and drier.
  • The inversion boundary varies its altitude primarily in response to the weight of the air above it which is measured in terms of atmospheric pressure.
  • Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred: Radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky.
  • It is most common at sea when moist air encounters cooler waters, including areas of cold water upwelling, such as along the California coast.